A mechanical seal is a seal that is formed by close proximity of two flat, annular, non-contacting seal faces, usually referred to as a static seal face and a rotating seal face, in that one face is typically static and sealed to a housing, while the other face is sealed to a rotating shaft and rotates with the shaft. The two seal faces are annular, and coaxial with the rotating shaft, whereby the long axis of the shaft is also the common axis of the annular seal faces.
Traditionally the two seal faces are lapped flat to a very high degree of flatness, which allows the faces to be configured in a parallel relationship whereby they are not physically in contact with each other, but are separated by a gap that can be as narrow as one micron or less. The gap itself is also annular in shape, and shares a common central axis with the two annular seal faces, and with the rotating shaft. Geometrically, the configuration of a mechanical seal is such that, if each of the annular seal faces were considered to lie in a plane, then the planes of the annular seal faces would be parallel to each other, and both would be perpendicular to the central axis of the rotating shaft.
The gap between the seal faces in a mechanical seal is filled with a film formed by fluid that is radially forced into the gap from either or both of the inner and outer boundaries of the gap due to differential pressure between the inner and outer boundaries. The film serves to lubricate the gap, and to maintain the separation of the faces so that they do not mechanically wear against each other. Often, but not always, this lubricating fluid is process fluid. It is inherent to mechanical seals that during operation a very small amount of the lubricating fluid leaks radially across the gap, i.e. from the inner boundary of the annular gap to the outer boundary of the gap, and/or vice versa. The amount of leakage varies according to the design and application, but a typical amount would be about one quarter of a teaspoon per hour. Generally, this is such a small amount that if the leaked fluid is a liquid, it tends to evaporate rather than pool, and therefore remains unnoticed.
Generally, it is not practical to permanently fix the seal faces to the shaft and housing. Instead, secondary seals are provided between the stationary seal face and the housing, and between the rotating seal face and the rotating shaft.
The effectiveness of a mechanical seal depends heavily on maintaining a very narrow gap between the seal faces. Typically, one of the seal faces is configured to be axially movable, and an axial force is applied thereto during operation so as to press it toward the other seal face and thereby compensate for wear, axial thrust, thermal expansion, and/or any other mechanism that might compromise the gap between the seal faces. Depending on the design of the seal, the axial thrust can be applied by a pressurized process fluid, and/or by a mechanical loading that is provided by a spring, bellows, or other pressing mechanism. This applied axial force is opposed by a hydrodynamic pressure of the lubricating fluid within the gap, thereby creating an equilibrium that maintains the small but finite gap size.
So as to accommodate the axial movements of the axially movable seal face, it is necessary for the associated secondary seal to be axially adaptive, in that it must maintain the secondary seal between the seal face and its associated structure (housing or rotating shaft) as the seal face is axially moved. Either of two types of axially adaptive secondary seal are typically used for this purpose, either a “pusher” secondary seal or a “non-pusher” secondary seal. Pusher secondary seals are secondary seals for which the element forming the seal is axially mobile. Typically, an O-ring is used to form a seal with a cylindrical element that is co-axial with the seal faces and is fixed to the housing or shaft, so that the O-ring is able to roll along the cylindrical element as the axially movable seal face undergoes axial movements. These O-ring pusher seals respond well to elevated pressures, due to the elastomeric deformation of the O-ring, which increases the contact area of the seal when the applied pressure is increased.
Non-pusher seals are secondary seals for which the element that forms the seal remains fixed in its attachment to the housing or shaft, and accommodates the axial movements of the associated seal face by expanding or contracting. In low temperature applications, an expandable, elastomeric gasket is sometimes used, whereas for high temperature applications a bellows is often used. In either case, the contact area of the seal does not typically increase with pressure for a non-pusher secondary seal.
During operation, a mechanical seal is necessarily subjected to both the temperature and pressure of the process fluid. For this reason, the use of a secondary seal that includes a gasket and/or O-ring can limit the maximum temperature at which a mechanical seal can operate, due to temperature limitations of the elastomer. Non-pusher secondary seals that use a metal bellows can withstand higher operating temperatures as compared to elastomer secondary seals, but metallic bellows seals are complex, expensive, difficult to install and remove, subject to metal fatigue, and are still sometimes not able to withstand the full range of temperatures that may be desirable for a given application.
What is needed, therefore, is a high temperature, high pressure secondary seal design that is simple in design, easy to install and remove, extends the operating temperature range of a mechanical seal beyond existing limits, and preferably facilitates axial movement of an associated seal face.